Gorgonian octocorals are often conspicuous components of benthic marine habitats from the deep sea to the tropical intertidal. Gorgonian ecology is - broadly defined but not limited to - their abundance, diversity, distribution, and relationship to their biological (i.e., other organisms including microorganisms) and physical environment. Figure 1. A colony of the zooxanthellate gorgonian Isis hippuris Linnaeus, 1758 amongst the seagrass beds of the Wakatobi Marine National Park, Indonesia. Image by SJ. Rowley.
Gorgonian ecology reflects together, or in part, the reproductive strategies and individual tolerances of species to the physical and biological changes that occur along environmental gradients. Environmental factors, such as substratum type and geomorphology, light, temperature, sedimentation, salinity, and water flow dynamics all influence gorgonian demography. Biotic factors, such as competition, predation, symbioses, reproduction, as well as settlement and development properties, further refine local community structure. Therefore, as with anything in life, the environment in which something is exposed will influence its form, function, and overall ability to survive.
Figure 2. Gorgonian corals across bathymetry (depth) on the reefs of the Senyavin Islands, FSM. (a) Viminella sp., at 65 m depth; (b) Subergorgia sp., populating the underside of overhangs and small caves at 12 m depth; (c) and inset, (d) Melithaea sp., in crevices and under the overhangs, 10 m depth. (e) Annella reticulata (Ellis & Solander, 1786) at 90 m depth. The zooxanthellate gorgonians (f) Rumphella sp., at 20 m, and (g) Briareum sp., at 38 m depth. (h) The depth-generalist genus Astrogorgia Verrill, 1868, pictured here at 75 m. (i) Paracis sp., at 110 m, and (j) Acanthogorgia sp., at 125 m. Deep-reef specialists (k) Heliania sp., at 106 m, and (l) Parisis sp. at 95 m. Images by SJ. Rowley (See Rowley et al. 2019).
Gorgonians are colonial sessile (i.e., not mobile) marine organisms that can be highly responsive (therefore plastic) to changes in their environment. This plasticity is because of their hierarchical modularity. However, the 'plastic' nature of many gorgonian taxa may render biodiversity assessments unreliable as the same species may look different in contrasting environments. Alternatively, they may be different species or even cryptic species, for example, where they look the same but are, in fact, not the same species at all! Without empirical evidence through quantitative data gathering and analyses, answers to these quandaries simply cannot be pursued in a satisfactory heuristic manner.
Gorgonian ecology (and ecology per se), therefore, provides the foundation for future study; to tease out these species-specific differences and responses, and the how and on what natural selection is working. Revealing ecological patterns can also assist with conservation management strategies as some phenotypes can be characteristic of a healthy or impacted environment (see Isis hippuris Linnaeus, 1758). By quantifying these observations and patterns particularly across environmental gradients such as depth, temperature, or reef health, it is possible to develop a conceptual framework to test mechanisms of adaptation such as plasticity as an adaptation, as well as divergence leading to enhanced biodiversity over ecological time.
Figure 4. Colonies of the zooxanthellate gorgonian Isis hippuris Linnaeus, 1758 on healthy reefs (left) and unhealthy (human-impacted) reefs of the Wakatobi Marine National Park, Indonesia. Images by SJ. Rowley (See Rowley et al. 2015, Rowley 2018).
Thus, ecological surveys highlight specific patterns in a highly complex system, enabling the system to be broken down to and reasoned from first principles. Thereafter, one can begin to elucidate what factor(s) or combination of factors are influencing species diversification and persistence over ecological and, eventually, geological time.
Figure 5. Species richness of gorgonian and scleractinian corals over depth (5–140 m) at the (a) atolls, and (b) Island of the Senyavin Islands. Red arrows indicate the transition depth between the two groups, whereby healthier reefs had a deeper (60 m) transition depth due to the zooxanthellate scleractinian corals being able to penetrate deeper on the atolls compared to the more human-impacted island, which had a slightly shallower transition depth (45 m). See Rowley et al. 2019.
The quantitative description of patterns then gives rise to theoretical model(s) as to why these patterns exist. Experimental ecology - a personal favourite - attempts to address these models through an iterative process of critically testing hypotheses, which are the predictions based on each model. Gorgonians are particularly excellent for experimental research because they are abundant, diverse, and pliable; and with so much unknown or misunderstood, there is no shortage of natural, biological phenomenon to be discovered, especially at depths below that of SCUBA.
As key benthic components of many marine habitats, gorgonian corals are ecologically diverse, long-lived engineering taxa that contribute to the biomass and, therefore, the energy flow of an ecosystem. Many aspects of their phenotype can be indicative of their trophic structure and the environment, past and present. The axi can inform of historical climatic events whereas the soft-tissue can reveal nutrient sources including pollution, as well as trophic level, and/or symbiotic dependence.
In sum, even though gorgonian corals are sessile (i.e., immobile) they are not passive to the vicissitudes of their surrounding environment. The can be ecosystem engineers, conservation flagship species (e.g., Eunicella verrucosa (Pallas, 1766); Isis hippuris Linnaeus, 1758), indicators of reef health and nutrient source, with symbiotic associations that are likely instrumental in the flow of energy (trophic ecolgy) that runs through the forests that these enigmatic creatures create. Yet, dispite their ecological importance and diverstiy, relatively little is known of gorgonian corals, particularly at mesophotic depths of the Indo-Pacific.
Rowley SJ, Roberts TE, Coleman RR, Joseph E, Spalding HL, Dorricott MKI (2019) Pohnpei, Federated States of Micronesia. In Loya Y, Puglise K, Bridge T (Ed.), Mesophotic Coral Ecosystems. Coral Reefs of the World, vol 12. Springer, Cham. DOI: 10.1007/978-3-319-92735-0_17
Sánchez JA, Dueñas LF, Rowley SJ, González FL, Vergara DC, Montaño-Salazar SM, Calixto-Botia I, Gómez CE, Abeytia R, Colin PL, Cordeiro RTS, Pérez CD (2019) Gorgonian Corals. In Loya Y, Puglise K, Bridge T (Ed.), Mesophotic Coral Ecosystems. Coral Reefs of the World, vol 12. Springer, Cham. DOI: 10.1007/978-3-319-92735-0_39
Rowley SJ (2018) Acclimatory capacity of the Gorgonian Isis hippuris Linnaeus, 1758 to environmental change in SE Sulawesi, Indonesia. Journal of Experimental Marine Biology & Ecology. 500: 73–88. DOI:org/10.1016/j.jembe.2017.12.012
Rowley SJ (2018) Environmental gradients structure gorgonian assemblages on coral reefs in SE Sulawesi, Indonesia. Coral Reefs. 37: 609-630. DOI:org/10.1007/s00338-018-1685-y
Rowley SJ, Pochon X. Watling L (2015) Environmental influences on the Indo-Pacific gorgonian Isis hippuris Linnaeus, 1758 (Alcyonacea: Isididae): genetic fixation or phenotypic plasticity? PeerJ 3:e1128; DOI:10.7717/peerj.1128
Rowley SJ (2014) Gorgonian responses to environmental change on coral reefs in SE Sulawesi, Indonesia. Doctoral thesis, Victoria University Wellington, New Zealand, pp. 213. DOI:10.13140/RG.2.1.5126.7682